Intrinsics are never intended to be stable directly, but intrinsics are often
exported in some sort of stable manner. Prefer using the stable interfaces to
the intrinsic directly when you can.

These are imported as if they were FFI functions, with the special
rust-intrinsic ABI. For example, if one was in a freestanding
context, but wished to be able to transmute between types, and
perform efficient pointer arithmetic, one would import those functions
via a declaration like